Sometimes it seems to take forever to learn things. And sometimes, we learn things in a relatively short time but it takes many years to master them.
I learned something important from a therapist. (Yeah, I had a therapist for clinical depression. It was good enough for Tony Soprano, so get over it.) This involves expectations, and what "should" be. He was fond of the Albert Ellis school of psychology, Rational Emotive Behavior Therapy. I don't agree with everything (I think the man's an atheist, for example), but a great deal of it makes sense.
Suppose I go into a shop. Normally, I expect professionalism and courtesy. But suppose this guy is having a bad day, and he's rude. He shouldn't be rude, he should treat me right. Isn't that awful? No, it's not. I get angry because I'm expecting something I'm not getting. Instead, I should accept the fact that it's a little thing. The world isn't going to cave in because he treats me like a cafone. Just dropping that expectation helps reduce the blood pressure and stress levels.
That dummy broad is touching up her makeup and talking on the cell phone while driving. She shouldn't be doing that, it's awful. No, she shouldn't. Drive around her or something, or tough it out. Again, no reason to get excited. Even the "dummy broad" judgment on her is an emotional investment that you're expending.
Some weird guy in the restaurant shouldn't be staring at me... OK, you work this example out for yourself.
We change what we can, sure. Certain injustices should be pointed out and rallied against, but the little day-to-day annoyances that cause us suffering can simply be acknowledged ("he was rude"), the emotions dealt with ("I don't like that") and be realistic ("I can try again, or find another shop"). One fellow used to say to me, "It ain't nothing but a thang".
"It is what it is", as the saying goes, and sometimes it's a useful saying. Something shouldn't happen, but guess what? It happened. We can't expect behavior from people. I made some really stupid mistakes expecting people to act or react in certain ways, putting those expectations on them and making them guess or perceive what I wanted. When I was able to realize what I was doing and really look at it, I saw the folly of my ways and dropped them. But I'm still learning, since emotions are not logical. They can be trained to respond to reason, however.
Interestingly enough, the Ellis approach, which is taking rational control of your emotions and your own choices, is also compatible with Buddhism. I do not accept all of Buddhism (I do have an immortal soul, Jesus did indeed bodily rise from the dead, etc.), but I have learned a great deal from it. One of the best things I got from it, and can mostly agree with (I don't see conflict with most religions in this, in fact) is a .pdf booklet on the Four Noble Truths.
You don't have to like something. But it's up to you how much fretting you're going to do. You can even influence just how depressed you're going to get over something happening that you don't like. Change it if you can, live with it otherwise. (In "Doctor Who", the Fourth Doctor said, "What can't be cured must be endured".)
But first, decide how much emotional energy you want to expend. Is it worth it? Start with the small stuff, even that helps the stress levels.